1. Field of the Invention
The present invention relates generally to a method of controlling wireless local area network medium access and, more particularly, to a method of controlling wireless local area network medium access using pseudo-time division multiplexing to improve the quality of service of Voice over Internet Protocol.
2. Description of the Prior Art
In general, a Wireless Local Area Network (WLAN) operates within 100 m in a transmission rate range of 10 to 100 Mbps. A WLAN composed of a single cell can be used in a single floor office or store. A WLAN terminal is connected to another terminal and an Access Point (AP) on a network via a Radio Frequency (RF) link using a wireless Network Interface Card (NIC).
An AP enables a WLAN terminal to access a wired network via a backbone network. Approximately 25 terminals are connected to a single cell. A multiple cell can be constructed using a plurality of APs that are connected to a wired network, and a WLAN environment can be built throughout the entire building using the multiple cell.
WLAN-related Institute of Electrical and Electronics Engineers (IEEE) standards are described below.
The IEEE developed a standard that defines a protocol regarding the transmission of a data frame between a WLAN terminal and an AP and, as a result, establishes a standard regarding Medium Access Control (MAC) and a Physical (PHY) layer (IEEE Std. 802.11, IEEE standard for Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY), 1999).
The IEEE 802.11 WLAN standard describes a mobile terminal and a fixed AP that are the two principal elements of a WLAN. A single cell using the IEEE 802.11 WLAN standard is defined as a Basic Service Set (BSS), and a multiple cell is defined as an Extended Service Set (ESS).
In the IEEE 802.11 WLAN standard, each terminal and each AP implements a MAC layer having a function capable of a MAC frame. The MAC frame is used as a medium for transmitting control and management data.
The IEEE 802.11 WLAN standard defines two different wireless medium access methods in the MAC layer: a Distributed Coordination Function (DCF) and a Point Coordination Function (PCF).
In the DCF, all the stations can participate in contention for the transmission of a frame. The basic access method of the 802.11 MAC is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). In CSMA/CA, a station that intends to transmit data to the wireless medium of a WLAN detects a medium to determine whether data transmission from some other station exists. If the medium is unoccupied, data transmission is performed; otherwise data transmission is delayed until ongoing data transmission is completed.
If data transmission from the station can be performed immediately after previous data transmission has been completed, transmission attempts from a plurality of stations may occur and, therefore, there is a high probability of data collision. In order to solve the problem, after a certain pause period is provided after the completion of data transmission, the size of a Contention Window (CW) is determined by performing binary random backoff, and a station that has the smallest determined CW size is provided with an opportunity to perform transmission. This process is called a Collision Avoidance (CA) function.
Meanwhile, in the PCF, a Point Coordinator (PC) controls transmission from WLAN terminals. The PC functions as a polling master, and polls all the PCF polling-capable terminals to determine the terminal that can perform data transmission. The PC may exist in the AP. In the PCF, a terminal may be capable of polling or not.
If a polling-capable terminal receives a poll from the PC, only a single MAC Protocol Data Unit (MPDU) can perform transmission. When additional transmission is required, waiting must be performed until a poll is received again. If specific data transmission is abnormally performed, a terminal may not retransmit until it receives a poll from the PC. Accordingly, the PCF provides a contention free mechanism to provide an opportunity for a terminal to normally transmit data.
Another wireless medium access method, that is, an Enhanced-DCF (EDCF) that conforms to IEEE 802.11e, intends to improve Quality of Service (QoS) by adjusting a CW. During a CW period, a plurality of stations contends for network access. In order to avoid a collision, a MAC protocol requests individual stations to wait for the CW period that is determined by binary random backoff. A probability of collision between stations is reduced due to the CW period that is determined by the binary random backoff. In this case, the EDCF employs the CW to grant higher priority to a specific station. Higher priority is granted to the specific station by providing a short CW to the specific station. As a result, in most cases, a higher priority station transmits data earlier than a lower priority station.
QoS is a measurement of service quality for a user. Principal measurements of QoS include message loss, message delay and network availability. The transmission of time-sensitive data application traffic (such as voice or video) on a packet network requires conditions that satisfy delay, delay jitter and error rate requirements.
When the wireless medium access methods DCF, PCF and EDCF are examined in terms of QoS, the DCF causes frame delay because it performs binary random backoff before the transmission of a frame, the EDCA supplementing the DCF can grant priority by providing a voice frame with a CW shorter than that which is provided to a data frame but causes frame delay because it still performs backoff, and the PCF causes frame delay because a WLAN terminal can transmit a frame only through polling during a contention free period.
In the meantime, U.S. Pat. No. 6,747,968 B1 entitled “Method and system for weighted PCK polling lists for WLAN QoS support” discloses a method and system for providing weights when an AP polls terminals on a WLAN. U.S. Pat Publ. No. 2004/0081133 A1 entitled “Method of communication device initiated frame exchange” discloses a method for accessing a WLAN channel and providing QoS for voice in a system supporting both voice and data services. However, the preceding patents are problematic in that they have limitations in ensuring a sufficient bandwidth for an RF link due to the performance of polling and transmission delay time is excessively long due to channel contention and polling.